Multihull watercrafts like the catamaran and the trimaran have been a popular alternative to the monohull version. The hulls are usually rigid and inactive. The arrangement of the hulls gives them good lateral stability. However, there is one type of very small watercrafts that have active hulls. A typical craft of this type has four very large lightly constructed plastic tires which resemble over-sized automotive tires. In fact, the craft""s arrangement of its tires is similar to that of an automobile. The tires are used for floatation and, with deep and wide crosswise grooves instead of the conventional random shallow grooves, are also used for propulsion as the tires are rotated by bicycle type foot paddle mechanisms. The buoyancy and propulsive ability of its tires qualify such craft as active hulls. Such crafts are lacking in propulsive efficiency because it is difficult to provide the tires with adequate water pushing area. The tires, with their horizontal axles, have a high center of gravity which in turn compromises stability. Such crafts are mostly used as playthings in calm water near beaches and small lakes in resort areas.
Crafts with active hulls are useful because hulls that can float and propel possess some unusual properties.
The following describes methods by which multihull watercrafts can be constructed to have good propulsive efficiency and substantially reduce hydrodynamic drag.
By referring to watercraft with active hulls described in the previous section, a brief summary of the invention can be presented more clearly and concisely by describing, in a step-by-step fashion, the changes and improvements achieved by the crafts in the present invention. The craft in prior art have two pairs of very large tire-shaped hulls with inadequate surfaces for efficient propulsion; their rotatable hulls, with their horizontal axles, have an excessively high center of gravity.
First, the present invention redesigns the tire-shaped hulls to each have their own individual axle with a power input end toward the center of the craft.
The second change rotates the tires and their axles upward approximately forty-five degrees, while holding the center of the tire-shaped hulls fixed.
At this point, a visualization of the front view of the craft will be helpful. From the front, only the front pair of the active hulls is visible. On each side of the vertical centerline of the craft lies a tire-shaped hull. Each tire-shaped hull with its axle is at a tilt, approximately forty-five degrees to a horizontal plane. The centerline of the axle of the right hull will intersect with the centerline of the axle of the left hull. The intersection is at the vertical centerline of the craft, above a horizontal line connecting the centers of hulls.
For simplicity, the frontal profile each tire-shaped hull can be represented by a rectangle. Since each tire-shaped hull is tilted approximately forty-five degrees, the rectangles representing them will also be tilted by the same angle. Since they are tilted, their total vertical height is reduced.
If a horizontal line representing the water level is placed somewhere above the lowest point of the tilted rectangle, the area of the rectangle(the outline of hull)is divided into two portions: one portion above water and one portion below water.
Further reduction of vertical height can be achieved by adjusting the proportions of the rectangle. The sides of the rectangle which are parallel to its axle can be lengthened until the rectangle becomes a square. The line of the rectangle nearest to axle""s power-input end and perpendicular to the axle is reduced in length, to lower the highest point of the rectangle. Since the length of the line of the rectangle nearest to axle""s power-input end is reduced, the ends of the two lengthened lines connected to it are brought closer together. At this point the rectangle has been transformed into a trapezoid.
The step to reduce the vertical height described in the last paragraph is not quite the last step. The trapezoid can be further refined by proportioning the trapezoid so the two inside angles adjacent to the trapezoid""s longest side are forty-five degrees. This is a configuration that yields a minimum vertical protrusion height above water.
The trapezoid represents only the outline of the hull. In three dimensions, the hull is a hollow truncated watertight cone, truncated by an annular top plate with a hole for the axle, and bounded by a larger circular base plate.
With the reduction of vertical protrusion height done, the matter of propulsion is now addressed. The external conical surface of the active hulls is the ideal place to attach a number of rectangularly shaped paddles to endow the hull with water pushing ability. The paddles are thin rectangular plates with one of their long edges attached to the conical surface. The long edge of the rectangular plates extend from the edge of the base plate to the edge of the top plate. The flat surface of the plates is parallel to the centerline of the of conical hulls. Future paddles may be curved if such paddles can achieve smoother operation. As the conical hulls rotate around their centerline, one paddle after another will dip into the water. As the conical hull rotates, the paddle""s wetted area increases until the wetted area reaches its maximum value when the paddle is the underwater vertical position. Water in contact with the wetted area on the advancing side of the paddles will be pushed; the water""s reaction on the paddles gives the paddles their propulsive power.
The propulsive efficiency is a function of the rotational speed of the hull, the number of paddles and the dimension of the paddle""s width. There is no problem expected in optimizing the three factors to achieve a good propulsive efficiency.
When the craft is operating on water, the hulls are partially immersed. The flat bases of the conical hulls are on the extreme right or left side of the craft. The bases are parallel to the direction of travel and are tilted upward to have an approximately forty five degrees with the surface of the water.
The base and the conical surface of each hull thus form a chisel shape as the hull moves through the water. An analysis of the water pushing action of the paddles and the special shape and orientation of the active hulls will reveal that a craft thus constructed can have a good propulsive efficiency and can attain a substantial reduction in drag.
Water normally resists the motion of passive hulls, the resistance showing up as frontal drag. In an active hull, this frontal drag is reduced by sweeping the water towards the rear with paddles. If the swept volume of the water is equal to the volume needed to move aside to the let the craft through, there will no frontal drag on the hull. In practice, some frontal drag will remain. A relatively small amount of energy expended by the paddles will move the water toward the center of the craft. This movement is not in the desired direction for propulsion. Later on, a method will be described for recovering part of this energy for forward propulsion.
The sweeping action requires the wetted portion of the conical surface of the hull to move rearward, on the average, almost as fast as the on-coming water. The conical surface of the hull thus encounter little friction drag.
The base plate of the rotating hulls, with its flat surface parallel to the direction of travel, will encounter no frontal drag but will encounter, on its wetted portion, some friction drag against its forward movement. The friction drag impeding forward movement at any point on the wetted surface is a function of the difference between the rearward speed of that point and the speed of travel. This speed difference varies as each point has a different radius measured from the center of rotation. At the lowest point of the base plate, the rearward speed is higher than the speed of travel so the speed difference is negative; the local friction drag is zero. The speed difference for the entire wetted surface has an average value that is smaller than the speed of travel. Since the friction drag is proportional to the one-and-a-half power of the speed difference, the rotation of the base plate substantially reduces substantially its friction drag compared to the friction drag on a stationary base plate.
The above orientation and rotational geometry enable the watercrafts with active hulls to attain a substantial reduction in hydrodynamic drag. So far, the descriptions and analyses are for a watercraft with two pairs of active hulls. A craft with one pair of active hulls and a central conventional hull can also have significant hydrodynamic advantages. Since the active hulls can carry a large fraction of the total weight, the central hull, with its reduced burden, will have less drag. A central hull can be designed to have surfaces to recover some of the energy wasted by the paddling of the rotating hulls. The arrangement of the hulls can be such that the bow waves made by the central hull are intercepted by the paddles of active hulls; energy in the waves can be recovered to boost the propulsive efficiency.